In general, tapes for data storage have some sort of indicator for optical detection of the beginning of the tape and the end of the tape. Indicators include transparent leaders and trailers, reflective leaders and trailers, reflective markers and various patterns of holes punched in the tape. A typical sensing apparatus uses a light source and a light-sensitive sensor to detect light reflected from a reflective leader/trailer/marker, light transmitted through holes or light transmitted through a transparent leader/trailer. In a typical sensing apparatus, the sensor generates a signal if light impinging on the sensor exceeds a fixed threshold. However, light reaching a sensor may vary for several reasons unrelated to the presence or absence of an indicator. Dust may accumulate on the light source or the sensor. Dust may partially clog a hole in the light path or one of the holes in the tape. Parts of the tape may be slightly transparent. The light source intensity or the sensor sensitivity may change with age or temperature. Various sources of ambient light may interfere. Finally, there may be inherent differences due to manufacturing tolerances for the light intensity, the sensor sensitivity, and other components in the light path.
Prior art approaches to the general problem have used continuous feedback to control the intensity of the light source, to control the gain of the sensor, or to filter the sensor signal. For example, U.S. Pat. No. 3,931,513, issued to Germain, discloses continuous analog feedback to control light intensity in a system using reflective markers. The type of tape used in Germain has a reflective marker on one edge of the tape for a beginning-of-tape indicator and a reflective marker on the opposite edge for an end-of-tape indicator. Separate sensors are used for beginning-of-tape and end-of-tape. Germain discloses a way to maintain a controlled light intensity even when a reflective marker is in the light path. When a reflective marker is in the light path, the sensor receiving the least light is used for feedback to control light intensity.
Similarly, in U.S. Pat. No. 4,410,148 issued to Dunlap, feedback is provided to control light intensity. In Dunlap, there is a time delay in the feedback so that when a reflective marker passes the sensor area, the light intensity is not changed immediately.
In U.S. Pat. No. 4,570,075, issued to Spiero, the light source is operated in a pulsed mode. By using filters, the high frequency light emitted from the pulsing light source is distinguished from lower frequency ambient light. In addition, the lower frequency ambient light component is used for continuous negative analog feedback to the sensor.
The prior art solutions provide analog feedback to control light intensity or sensor sensitivity. Lower cost solutions are needed which take advantage of the control and flexibility of on-board microprocessors. With low cost microprocessors and digital circuitry available in Application Specific Integrated Circuits (ASIC), a digital approach can provide a lower cost solution and fewer parts. Alternatively, for an analog solution, the microprocessor can be used to simplify the analog circuitry.